Electrochromism is a property displayed by some materials wherein the materials reversibly change color when an electronic charge is applied. Electrochromism occurs due to electrochemical redox reactions that take place at certain positions in the electrochromic materials. Various types of materials and structures can be used to construct electrochromic devices, depending on the specific applications
Transition metal oxides represent a large family of materials possessing electrochromic properties. Among them, tungsten oxide, has been widely used in such applications as “smart glass”.
Many organic materials exhibit redox states with distinct electronic (UV/visible) absorption spectra, wherein the switching of redox states generates new or different visible region bands. Color changes are commonly between a transparent, bleached state, where the chromophore only absorbs in the UV region, and a colored state. The switching may also be between two colored states.
Often, when there are more than two redox states which are electrochemically accessible in a given electrolyte solution, the electrochromic material may exhibit several colors and can be termed polyelectrochromic. As might be expected from the enormity of the field of organic chemistry, there are a vast number of organic compounds that exhibit electrochromism. Examples of well applied organic electrochromic materials are the viologens and conducting polymers such as polyacetylenes, poly(3-alkylthiophene)s, poly(3,4-alkylenedioxythiophene)s, often coupled with poly(styrenesulfonate), and polyaniline. Depending on the oxidation state polyaniline, for example, can be pale yellow, dark green, purple or black. Polymers based on triarylamine or carbazoles have also been used as organic electrochromic materials. Some organometallic materials such as metallopolymers and metallophthalocyanine also exhibit electrochromic behavior.
Iptycenes are cyclic materials that are built upon [2,2,2]-ring systems in which the bridges are aromatic rings. The simplest member of this class of compounds is triptycene, FIG. 1A. Iptycenes are unique materials in that they can provide steric blocking, which prevents interactions between various substituents of different molecules. This is especially important in chromophoric materials as interaction between chromophores that have a strong tendency to form non-emissive exciplex complexes when they interact is prevented or at least reduced. Iptycene materials are very stable and display solution-like emissive spectra and quantum yields in the solid state.
The three-dimensional shape of iptycenes creates interstitial space, or free volume, around the molecules. This space can confer size selectivity in sensory responses and also promotes alignment in oriented polymers and liquid crystals. Specifically, the iptycene containing polymers and molecules align in the anisotropic host material in a way that minimizes the free volume.
Structures that preserve high degrees of internal free volume can also be designed to create low dielectric constant insulators. These materials have high temperature stability (>500° C.) and hardness that make them useful in high temperature applications where other materials would not function properly.
The iptycene structures allow for small interpolymer motions, but at large deformations, the steric interactions between iptycenes result in the transfer of stress energy from one polymer to another. This mechanism has the ability to impart greater modulus, strength, and ductility. The use of iptycene structures is a promising approach to new generations of structural materials.
Thus there is an unmet need for electrochromic materials which provide a wide range of colors while having improved properties in solution and in applications.